Conditioning of roots with hypoxia increases aluminum and acid stress tolerance by mitigating activation of K+ efflux channels by ROS in barley: insights into cross-tolerance mechanisms

Aluminum (Al) is prevalent in soils, but Al toxicity is manifested only under acid conditions. It causes severe damages to the root system. Short-term waterlogging stress can occur simultaneously with Al toxicity in areas with high rainfall or an inappropriate irrigation pattern. Barley (Hordeum vulgare L.) is one of the most Al-sensitive small-grained cereals. In this work, we have investigated effects of short-term treatments with hypoxia and phenolic acid (two major constraints in waterlogged soils) on root sensitivity to low-pH and Al stresses. We showed that hypoxia-primed roots maintained higher cell viability when exposed to low-pH/Al stress, in both elongation and mature root zones, and possessed superior ability to retain K⁺ in response to low-pH/Al stresses. These priming effects were not related to higher H⁺-ATPase activity and better membrane potential maintenance, and could not be explained by the increased expression levels of HvHAK1, which mediates high-affinity K⁺ uptake in roots. Instead, hypoxia-conditioned roots were significantly less sensitive to H₂O₂ treatment, indicated by the 10-fold reduction in the magnitude of K⁺ efflux changes. This suggested that roots pre-treated with hypoxia desensitized reactive oxygen species (ROS)-inducible K⁺ efflux channels in root epidermis, most probably via enhanced antioxidative capacity. A possible role for Ca²⁺ in stress-induced ROS signaling pathways is also discussed. Overall, our results report, for the first time, the phenomenon of cross-protection between hypoxia and low-pH/Al stresses, and causally link it to the cell’s ability to maintain cytosolic K⁺ homeostasis.